This invention relates generally to semiconductor diodes and lasers and, more particularly, to semiconductor lasers of the vertical-cavity surface-emitting type. Most semiconductor diode lasers are designed to emit light from an edge of a layered structure, in a direction parallel to the plane of an active layer in the device. In such a device, light is confined substantially within the active layer, and a Fabry-Perot cavity is formed between a pair of cleaved facets at end faces of the structure. However, for some applications of semiconductor lasers it would be desirable to have the light emitted in a direction perpendicular to the planar layers of the device. This avoids the need for a separate processing step to form cleaved facets, and reflectors can instead be formed as part of an epitaxial fabrication process. Circuit chips containing surface-emitting devices and formed on a larger semiconductor wafer, can be probe tested without having to break the chips from the wafer. Also, surface emitting structures facilitate coupling of the emitted light to optical fibers and other components.
Previous attempts to construct surface-emitting diode lasers have not been successful in achieving relatively high powers. Some previously described surface emitting structures have used a dielectrically defined metallic contact mirror configuration located behind the light source in the active layer, and a high-reflectivity dielectric-stack or semiconductor-stack reflector located in front of the source. The resultant external differential quantum efficiencies of these devices have been limited to approximately the 3-5% range, and output powers have been thermally limited to approximately 10 milliwatts (mW).
A surface-emitting diode laser structure is disclosed in U.S. Pat. No. 4,309,670 issued to Burnham et al. In that structure, current and carrier confinement to the vertical cavity is enhanced by a confinement region of different semiconductor materials, laterally surrounding an active region of the device. In other words, the active region of the device is buried both transversely, by surrounding planar layers, and laterally, by other semiconductor materials in the confinement region. One of the disclosed embodiments of Burnham et al. has a distributed Bragg reflector as one of the end mirrors of the vertical cavity. One significant drawback to the structures disclosed by Burnham et al. is that the active region cannot be located near a heat sink, and sustained operation in continuous-wave (CW) mode is therefore impossible. CW operation is important because the vast majority of applications of diode lasers of the same general type as the invention are required to operate in a highspeed modulation mode that is practically equivalent to CW operation.
It will be appreciated from the foregoing that there is still a significant need for improvement in the field of vertical-cavity surface-emitting diode lasers. In particular, there is a need for a surfaceemitting laser diode that can operate at much higher powers and quantum efficiencies than were previously attainable. The present invention satisfies this need.